Compliant plate seals are used to provide dynamic sealing between a rotor, for example a rotating shaft, and a stator, for example a static shell, casing or housing. Compliant plate seals are used in turbo-machinery to provide good sealing, non-contacting operation and high pressure capability. Examples of compliant plate seals are disclosed, for example, in U.S. Pat. Nos. 6,343,792 and 6,976,680.
In the leaf seal design discussed above, the geometry requires that if the leaves are packed tightly at the tips, gaps will remain between the leaves close to the seal root. These gaps offer an increased leakage path from the high pressure side to the low pressure side of the seal.
It is also known to radially position seal segments in a circumferential array between the rotor and the surrounding casing of the turbine to minimize leakage. Labyrinth packing rings and springs holding the segments radially inward against surfaces on the casing to establish radial clearance between the seal and the rotor are conventionally used to allow segments to move radially outward in the event of rotor contact. However, labyrinth seal performance degrades over time as a result of transient events in which the stationary and rotating components interfere, rubbing the labyrinth teeth and permanently opening the seal clearance. It is known to reduce the degradation due to rubbing by employing “positive pressure” variable clearance labyrinth packings in which springs are used to hold the packing ring segments open under the no or low flow conditions when such rubbing is most likely to occur. Ambient pressure forces overcome the springs at higher loads acting to close the rings to a close running position. Such variable clearance positive pressure packings are disclosed, for example, in U.S. Pat. Nos. 6,715,766 and 6,695,316.
Adjustable seals are also known that utilize a series of high pressure fluidic actuators to move the seal segments away from the rotor in case of imminent seal/rotor rub. An external high pressure air supply and control system is provided that feeds high pressure actuation fluid into the actuators. The actuators are disposed between the stator, for example, the packing casing, and the seal segments, for example, the packing ring segments, such that pressurization of the actuators leads to the opening or retraction of the seal segments. In the absence of actuator pressurization, ambient pressure in the machine forces the seal segments to close. Examples of such adjustable seals include, for example, U.S. Pat. Nos. 6,502,823 and 6,786,487.
During periods of turbine startup and shutdown, there is minimal, or no, pressure drop across the seal, and the compliant plate seals must rely on hydrodynamic lift for non-contact operation. Contact between the compliant seal and the rotor causes seal and rotor wear, heat generation, and rotor-dynamic instabilities.